Concrete ears that intercepted enemy aircraft now form natural bird habitat

Watch: Bird song is now amplified by the acoustic mirrors designed to capture the sounds of aircraft.

The sound mirrors, also known as acoustic mirrors or listening ears, are large concrete structures that once captured sound to detect incoming enemy aircraft before it became visible.

Today the giant concrete mirrors of which there were three designs – 20ft, 30ft and 200ft tall – are part of a Royal Society for the Protection of Birds (RSPB) Nature Reserve.

Forces News was given an exclusive tour of the former RAF Denge site by its current custodians – the RSPB Nature Reserve staff.

The sound mirrors were a massive success, able to hear an aeroplane engine far earlier than ever before.

RAF personnel were not only aware of when an aircraft was incoming but were also able to locate it.

"The shock of the First World War was the advent of the aircraft and from the beginning of the First World War to the end of the first World War, the realisation that an enemy could reach out and bomb your capital city," said RSPB Volunteer Peter Osborne.

"So you had to build an effective means of detecting them inbound," he added.

By the beginning of the Second World War, progress was catching up with these effective but low-tech solutions.

In the interwar period military aviation underwent a complete transformation.

The typical combat aircraft of 1918 was a fabric-covered biplane with fixed landing gear and open cockpits.

By 1939, the first-line combat aircraft of the major powers had retractable landing gear and were made of metal.

The rapid technological developments in the 20 years between the First and Second World Wars meant that aircraft could fly higher, further, and much faster.

The sound mirrors simply could not keep up with the pace of technological development, with planes becoming too fast to be effectively intercepted.

Sound mirror CREDIT BFBS
A sound mirror at the former RAF Denge.

During the First World War few aeroplanes had as much as 250 horsepower and could fly at a top speed of 120mph (200kph).

By 1939, combat aircraft had engines with 1,000hp and could easily fly at speeds of 350mph (560kph).

Some bombers flew faster than 400 km (250 miles) per hour, rendering the sound mirrors useless.

"What killed it essentially was the speed of the aircraft, because if you can only detect out 10 to 20 miles, then when an aircraft still in 330 miles an hour, as opposed to a Zeppelin maybe doing 60 miles an hour, you're not going to get much warning," Mr Osborne said. 

The listening devices were surrounded by gravel which has been extracted over the years.

Once filled in with water the gravel pits became a perfect environment for birds. The reserve is teeming with wildlife with more than 3,000 species.

According to Craig Edwards, the warden at RSPB Dungeness, the site is one of the most important places for wildlife conservation in the country.

The mirrors still serve their original purpose, but for ornithological rather than aeronautical identification. Now, the mirrors amplify bird sounds rather than incoming enemy aircraft.

"The sound reflections that you get from the mirrors are from the wildlife themselves. So you can sit here and you can hear birds quite a long way away because the sounds are coming back in from the mirrors," Mr Edwards explained. 

Despite the challenge of keeping up with rapid aircraft development, the technology at RAF Denge was utilised in the Second World War.

The sound locators were used to set guns in the direction of enemy places.

Today, the experimentation of the past, which tried to do what radar does so successfully for us now, stands strangely at home in a wildlife haven.

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